Electromagnetic security locks are well known in which an electromagnet is mounted to one of a door frame or a door and an armature is mounted to the other. In the predominant installations of such locks, the armature and electromagnet are generally mounted so that the face of the electromagnet and armature is parallel with a plane of the door and the holding force or attractions between the electromagnet and armature is perpendicular to the door when in a closed position. Any attempted forced opening of the door is resisted by the electromagnetic attraction of the armature to the electromagnet.
This type of arrangement is very effective, providing a locking force against unauthorized opening of a door. However, as is generally the case, the armature must be mounted on the vertical surface of the door, while the electromagnet is mounted from the door frame and overhangs the top edge of the door. This type of lock, while very effective from a security standpoint, is not suitable for mounting on many types of swinging or double-acting doors.
Also, in many instances, for aesthetic purposes, a better concealment of the electromagnetic lock is desirable. This has led to increased popularity of the so-called magnetic shear lock, in which the electromagnet is mounted within the door frame and an armature is mounted at or adjacent the top edge of the door and is adapted to be attracted to the electromagnet when the door is in a closed position.
This type of magnetic shear lock presents some technical problems which must be considered. When such a magnetic shear lock is mounted to a door and frame such that the electromagnet will exert an upward pull on the armature to achieve locking of the door, there must be some mechanical locking effort because the full magnetic attraction of the armature to the electromagnet is not available as a holding force when attempt is made to produce shearing movement between the armature and the electromagnet.
This problem has been recognized in U.S. Pat. No. 4,562,665, which discloses an armature mounted in the door which has two projections with vertical side walls which fit into two mating recesses in the electromagnet. Thus, when the armature is attracted to the electromagnet, the projections extend into the recesses and provide a mechanical lock. This type of construction does not take into account the possible effects of residual magnetism in the electromagnet when it may be deenergized to permit opening of the door, particularly if an opening force is applied to the door at the same time the electromagnet is deenergized. Specifically, if there is an attempt being made to open the door when the electromagnet is deenergized, the residual magnetism may still provide attraction between the armature and the electromagnet, and the vertical surfaces of the projections in the recesses will prevent opening of the door.
This problem appears to have been recognized in U.S. Pat. No. 4,487,439 in which a non-magnetic projection is placed in one of the armature and the electromagnet, and a recess placed in the other, and a beveled edge is defined on the projection to enable the armature to be pushed away from the electromagnet by a force attempting to open the door when the electromagnet is deenergized. In this construction, the projection has a matching circular indentation or recess somewhat larger in diameter than the projection. However, this type of construction suffers from a drawback in that when an attempt is made for unauthorized opening of the door, the circular pin or projection only makes a point contact with the recess in the armature, and this will very quickly show signs of wear after attempts to force the lock by applying shear force.
The problem of the residual magnetism in an electromagnetic shear lock which includes a mechanical reinforcing structure of the electromagnet when energized is discussed in U.S. Pat. No. 4,439,808. In the construction disclosed in this patent, a magnetic flux cancelling circuit is provided to overcome the residual magnetism in the electromagnet when the electromagnet is switched off.
Accordingly, the present invention provides a new and improved magnetic shear lock having mechanical holding means when the electromagnet is energized, which is so constructed that the effects of residual magnetism will not prevent opening of the door when the electromagnet is deenergized while an opening force is applied to the door.